This invention relates to electrophoretic displays, methods for driving such displays and electrophoretic media for use in such displays.
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles dispersed in a solvent. The display typically comprises two plates with electrodes placed opposing each other. One of the electrodes is usually transparent. An electrophoretic medium composed of a colored fluid with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the fluid being seen from the viewing side.
Alternatively, an electrophoretic medium may comprise two (or more) types of charged pigment particles of contrasting colors and carrying opposite charges, and the two types of the charged pigment particles are dispersed in a clear fluid. In this case, when a voltage difference is imposed between the two electrode plates, the two types of the charged pigment particles would move to opposite sides. Thus one of the colors of the two types of the charged pigment particles would be seen at the viewing side.
The aforementioned patents and published applications describe an electrophoretic medium containing three types of particles, typically all having differing optical characteristics. (Although the optical characteristic is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.) The first and second types of particles carry charges of opposite polarities. The third type of particles carries a charge of the same polarity as the second type of particles but has a lower zeta potential and/or electrophoretic mobility than the first or second types of particles. By careful control of the magnitude and sequencing of the electric fields applied to such an electrophoretic medium, the optical characteristics of the three types of particles can be displayed at a viewing surface of a display. As illustrated for example in FIG. 2 of the aforementioned U.S. Pat. No. 8,717,664, the optical characteristics of the first and second types of particles are displayed by applying high electric fields of the appropriate polarity to the electrophoretic medium in essentially the same way as in the two particle media discussed in the preceding paragraph. To display the optical characteristic of the third type of particles, one first drives the medium to display the optical characteristic of the first type of particles (i.e., the particles bearing a charge of the opposite polarity to the third type of particles) and then applies a low electric field of a polarity which tends to drive the third type of particles towards the viewing surface. This low electric field causes the first type of particles to move away from the viewing surface and the second and third type of particles to move towards the viewing surface. However, it appears (although the present invention is in no way limited by this belief) that the highly charged first and second types of particles aggregate with each other, so that only the third type of particles are driven adjacent the viewing surface, so that the optical characteristic of the third type of particles is displayed.
One problem with conventional electrophoretic media is that they cannot be driven using passive matrix driving methods (in which an electric field is applied by means of a first set of elongate electrodes disposed on one side of the electrophoretic medium and a second set of elongate electrodes are disposed on the opposed side of the electrophoretic medium, the first and second sets of elongate electrodes being arranged with their long axes at an angle to each other so that a plurality of pixels are formed, each pixel being defined by the overlap of one electrode of the first set with one electrode of the second set). Passive matrix driving methods require that the electrophoretic medium have a substantial threshold voltage (i.e., a voltage which can be applied across the medium for a prolonged period without causing any change in the optical state displayed) and most conventional electrophoretic media have no threshold voltage or only a minimal one. It has now been discovered that a subset of the three particle electrophoretic media described in the aforementioned patents and published applications can be driven by passive matrix driving methods, and the present invention relates to such media, electrophoretic displays incorporating such media, and driving methods for use in such electrophoretic displays.